task.source doesn't take effect after a yeild task

Nobody has claimed this yet.

I write a simple build task and a custom plugin `usemin` as following.
I thought the `files` argument of the generator function in the `usemin` plugin should be `.tmp/index.html` but, I found that I'm wrong, it contains all the files which returned by the `html` task.

so why things become so weird? (maybe I should post this question on stackoverflow. sorry ....

```js
export async function compile(task) {
await task.start('clear');
await task.parallel(['script', 'style', 'image', 'font', 'other', 'html']);
}
```
```js
export async function build(task) {
await task.start('compile');
await task.source('.tmp/index.html').usemin({
staticDir: c.build,
}).target(c.build);
}
```

```js
module.exports = function (task, utils) {
// ...
task.plugin('usemin', { every: false }, function* (files, opts) {
console.log(files.map(o=>o.dir).filter(o=>o.indexOf('.tmp')!==-1));
// [] (empty array)
});
}
```

Start by reproducing the compile and build task sequence shown in the issue, especially task.start('compile') followed by task.source('.tmp/index.html'). Inspect the usemin plugin callback's files argument and compare it with the expected source; done means determining whether the retained files are intended behavior or a task-runner bug.

Written by the indexing model from the issue text.